Normative neurogenesis in mammals will be studied at the levels of light and electron microscopy. For the determination of times of origin, sites of origin and paths of migration of various neuronal elements the primary technique employed will be thymidine-H3 autoradiography. These studies will be conducted on the rat and rabbit. For the determination of times of origin of neurons embryos will be injected with thymidine-H3 at different stages of development and will be sacrificed 2-3 months after their birth. For the determination of sites of origin and paths of migration of various neurons, embryos will be administered thymidine-H3 on different stages of development and will be sacrificed at one day intervals. The objective of this research activity is to study neurogenesis at cytological level of the entire central nervous system from a comprehensive viewpoint. Experimental studies will involve transplantation of embryonic neuroblasts and of precursors of nerve cells in the central nervous system of neonate rats and rabbits. These studies will aid us in resolving the following issues: specificity of neuronal differentiation, and specificity of cytoarchitectural evolution in the transplanted neuronal tissue; development of synapses and their nature in the transplanted neuronal elements; and cellular interactions between the transplanted and host nerve cells. BIBLIOGRAPHIC REFERENCES: Das, G. D. Differentiation of Bergmann glia cells in the cerebellum: A Golgi study. Brain Research, 110: 199-213 (1976). Das, G. D. Resting and reactive macrophages in the developing cerebellum: An experimental ultrastructural study. Virchows Archiv B. Cell Pathology, 20: 287-298 (1976).